The presence and characteristics of a lipid core are important considerations for assessing the risk of a coronary artery plaque. For a lipid core plaque (“LCP”), the thickness of the fibrous cap overlying the lipid-filled core is widely regarded as an important indication of the LCP's risk of rupture. LCPs displaying characteristics such as expansive remodeling, increased plaque volume, inflammation, lipid core, and cap thicknesses below, for example, approximately 0.065-0.1 mm can be classified as thin-capped fibroatheromas (“TFCAs”). TCFAs are the histopathologically-defined structures thought to be associated with in vivo vulnerable plaques and are the structures most often implicated, post-mortem, as the culprit site in sudden coronary deaths. As a result, the detection of LCP cap thickness in vivo is of great interest to the interventional cardiology community as a tool to assess future risk of LCP rupture.
Few existing techniques have sufficient capability to measure cap thickness, and the existing techniques capable of measuring cap thickness have deficiencies. For example, intravascular ultrasound (“IVUS”) lacks adequate resolution to visualize the thickness of fibrous caps. IVUS is often utilized to estimate the presence of a thin cap overlying a plaque. Some approaches utilize definitions by which the apparent visual absence of a cap overlying a plaque is assumed to mean the cap thickness is below the typically-stated resolution limit of 40 MHz IVUS (100 μm). For example, in some IVUS approaches if echolucent plaque regions thought to be lipid cores seem to be in communication with the lumen, then the cap thickness is assumed to be below 100 μm, and if the plaque burden is also elevated, the structure is assumed to be an in vivo TCFA.
As another example, optical coherence tomography (“OCT”) has adequate resolution (variously reported as tens of microns) to visualize the thickness of a fibrous cap. However, the accuracy of OCT techniques can be adversely affected by numerous image artifacts, and consensus recommendations for the inherently subjective offline interpretation of multiple cross sections per plaque have yet to be developed.